Sound can be broken down into two categories; diegetic and non diegetic sound. Diegetic sound can be defined as sound which occurs within the world of the film and therefore is heard by the characters. The source of the dietetic sound can come from something onscreen or off screen. Non diegetic sound however, is sound which does not occur within the narrative and therefore not heard by the characters. The most common form of this is music which has been added into the scene to help establish the mood and atmosphere. 

As well we setting the scene, sound can also direct the viewers attention to what is most importance. For example, a scene talking place in the heart of a busy city may start with an establishing shot. In this shot the audience hears people walking on the street, traffic noise, people talking. The next shot shows a couple sitting at a cafe having a conversation. The background noise fades away and their conversation is the main sound the audience hears. Therefore, this change in the diegetic sound cues the audience to focus on the couple talking. This example is also good for considering sound perspective. It is expected that the audience would hear the street noise with the establishing shot. But when it cuts to a medium or close up of the couple talking, it is assumed that because the audience is positioned closer to them, their conversation would be the dominant sound heard. Therefore sound is a useful tool in establishing setting, mood, and relative distance.